This application claims the benefit of Korean Application Nos. 00-52364, filed Sep. 5, 2000 and 00-52365, filed Sep. 5, 2000, in the Korean Patent Office, the disclosures of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium battery, and more particularly, to a lithium battery having improved safety and reliability by using a gel electrolyte.
2. Description of the Related Art
Lithium secondary batteries generate electricity by lithium ions migrating between a cathode and an anode. The lithium secondary batteries have higher energy density per volume and have a higher voltage, than lithium cadmium batteries or nickel hydrogen batteries. Also, the lithium secondary batteries are lightweight, compared to lithium cadmium batteries or nickel hydrogen batteries, that is, approximately one half as heavy. Thus, the lithium secondary batteries are well adapted for miniaturization and long-time use in electronic appliances.
As described above, since the lithium secondary batteries have higher voltage characteristics and better charging/discharge cycle life than conventional nickel cadmium batteries or nickel hydrogen batteries, without causing environmental problems, much attention has been paid thereto as the most promising high-performance batteries. However, it is a critical issue to attain safety due to danger of explosion of a lithium secondary battery.
Lithium secondary batteries are classified according to the kind of electrolyte used, into lithium ion batteries and lithium ion polymer batteries. The lithium ion batteries generally use a cylindrical case or prismatic case as the case for sealing an electrode assembly. However, recently, a pouch has been in widespread use instead of such a case. The reason of using a pouch as a case is that the energy density per weight and volume increases, thin and lightweight batteries can be attained, and the material cost for the case can be reduced.
FIG. 1 is an exploded perspective view showing an example of a lithium ion battery using a pouch as a case.
Referring to FIG. 1, a lithium ion battery includes an electrode assembly 10 having a cathode 11, an anode 12 and a separator 13, and a case 14 surrounding and hermetically sealing the electrode assembly 10. Here, the electrode assembly 10 is formed such that the separator 13 is interposed between the cathode 11 and the anode 12 and wound. A cathode tap 15 and an anode tap 15xe2x80x2, serving as electrical passageways between the electrode assembly 10 and the outside, are drawn from the cathode 11 and the anode 12, respectively, to form electrode terminals 16 and 16xe2x80x2.
FIG. 2 is an exploded perspective view showing an example of a conventional lithium ion polymer battery.
Referring to FIG. 2, the lithium ion polymer battery includes an electrode assembly 21 having a cathode, an anode and a separator, and a case 22 surrounding and hermetically sealing the electrode assembly 21. Electrode terminals (or lead wires) 24 and 24xe2x80x2, serving as electrical passageways for inducing the current formed at the electrode assembly 21 to the outside, are connected to a cathode tap 23 and an anode tap 23xe2x80x2 provided at the cathode and the anode, respectively, and are exposed outside the case 22 by a predetermined length.
As described above, in the lithium ion battery shown in FIG. 1 and the lithium ion polymer battery shown in FIG. 2, the electrode assemblies 10 and 21 are put into the cases 14 and 22 and an electrolytic solution is injected therein, with the electrode terminals 13 and 13xe2x80x2 and 24 and 24xe2x80x2 being partially exposed. Then, heat and pressure are applied so that the thermally adhesive materials in the edges of upper and lower case parts are adhered to be sealed, thereby completing the battery.
As described above, since the electrolytic solution is injected during a subsequent process to the one in which the electrode assemblies are put into the cases, in the case of using an organic solvent having a low melting point, the electrode assembly or pouch may swell, resulting in deterioration in the reliability and safety of the battery.
To solve the above-described problems, there have been proposed several methods of fabricating batteries such that plain batteries are cured by UV rays or electron beams, or electrode plates are coated with gel without separately injecting an electrolytic solution, as disclosed in U.S. Pat. Nos. 5,972,539, 5,279,910, 5,437,942 and 5,340,368. In practice, swelling of an electrode assembly or pouch can be somewhat mitigated, which is, however, not yet satisfactory.
To solve the above problems, it is a first object of the present invention to provide a polymeric gel electrolyte which can effectively suppress swelling of a battery due to an electrolytic solution.
It is another object of the present invention to provide a lithium battery which has improved reliability and safety by employing the polymeric gel electrolyte.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects, there is provided a polymeric gel electrolyte prepared by curing a composition comprising a polysiloxane compound represented by formula 1 or a polysiloxane-polyoxyalkylene compound represented by formula 2, a polyethylene glycol derivative represented by formula 3, and an organic solvent containing a lithium salt:
[Formula 1]
R1xe2x80x94Oxe2x80x94(CH2)mxe2x80x94[Si(R2R3)xe2x80x94O]nxe2x80x94Si(R2R3)xe2x80x94(CH2)nxe2x80x94Oxe2x80x94R4 
[Formula 2]
R5xe2x80x94Oxe2x80x94(CH2)mxe2x80x94[Si(R2R3)xe2x80x94O]nxe2x80x94Si(R2R3)xe2x80x94(CH2)mxe2x80x94Oxe2x80x94(CH2xe2x80x94CHR6xe2x80x94Oxe2x80x94)xxe2x80x94R7 
[Formula 3]
H2Cxe2x95x90C(Rxe2x80x2)C(xe2x95x90O)(OCH2CH2)n-ORxe2x80x3
wherein R1 is xe2x80x94C(xe2x95x90O)CR8xe2x95x90CR9R24 or xe2x80x94CR10R11R25, R2 and R3 are independently one selected from the group consisting of CaH(2a+1) in which a is an integer from 1 to 5, phenyl, benzyl and allyl, R4 is one selected from the group consisting of xe2x80x94C(xe2x95x90O)CR12xe2x95x90CR13R14, xe2x80x94CR15R16R17 and xe2x80x94CaH(2a+1) (here, a is an integer from 1 to 5), R5 and R7 are independently xe2x80x94C(O)CR18xe2x95x90C(R19R20) or xe2x80x94CR21(R22R23), R6 is hydrogen or xe2x80x94CaH(2a+1) (here, a is an integer from 1 to 5), m is an integer from 1 to 5, n is an integer from 1 to 20, x is an integer from 1 to 15, and R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, R18, R19, R20, R21, R22, R23, R24 and R25 are independently hydrogen or xe2x80x94CbH(2b+1) in which b is an integer from 1 to 5, Rxe2x80x2 is hydrogen or CH3, and Rxe2x80x3 is hydrogen, xe2x80x94C(xe2x95x90O)CHxe2x95x90CH2 or xe2x80x94C(xe2x95x90O)C(CH3)xe2x95x90CH2.
The polyethylene glycol derivative represented by formula 3 is at least one selected from the group consisting of polyethylene glycol monomethacrylate, polyethylene glycol dimethacrylate, polyethylene glycol monoacrylate and polyethylene glycol diacrylate. Also, the composition may further include ethoxylated trimethylolpropane triacrylate. In the composition, the content of the polysiloxane compound represented by formula 1 or the polysiloxane-polyoxyalkylene compound represented by formula 2 is preferably 0.1 to 10 parts by weight based on 100 parts by weight of the composition. The content of the polyethylene glycol derivative represented by formula 3 is preferably 0.4 to 50 parts by weight based on 100 parts by weight of the composition. The content of the ethoxylated trimethylolpropane triacrylate is greater than 0 parts by weight and less than or equal to 5 parts by weight based on 100 parts by weight of the composition.
To achieve the above and other objects of the present invention, there is provided a lithium battery including an electrode assembly having a cathode, an anode and a separator interposed between the cathode and the anode, a gel electrolyte formed by curing a composition comprising a polysiloxane compound represented by formula (1) or a polysiloxane-polyoxyalkylene compound represented by formula (2), polyethylene glycol derivative represented by formula (3), and an organic solvent containing lithium salt, and a case accommodating the electrode assembly and the gel electrolyte:
[Formula 1]
R1xe2x80x94Oxe2x80x94(CH2)mxe2x80x94[Si(R2R3)xe2x80x94O]nxe2x80x94Si(R2R3xe2x80x94(CH2)nxe2x80x94Oxe2x80x94R4 
[Formula 2]
R5xe2x80x94Oxe2x80x94(CH2)mxe2x80x94[Si(R2R3)xe2x80x94O]nxe2x80x94Si(R2R3)xe2x80x94(CH2)mxe2x80x94Oxe2x80x94(CH2xe2x80x94CHR6xe2x80x94Oxe2x80x94)xxe2x80x94R7 
[Formula 3]
H2Cxe2x95x90C(Rxe2x80x2)C(xe2x95x90O)(OCH2CH2)n-ORxe2x80x3
wherein R1 is xe2x80x94C(xe2x95x90O)CR8xe2x95x90CR9R24 or xe2x80x94CR10R11R25, R2 and R3 are independently one selected from the group consisting of CaH(2a+1) in which a is an integer from 1 to 5, phenyl, benzyl and allyl, R4 is one selected from the group consisting of xe2x80x94C(xe2x95x90O)CR12xe2x95x90CR13R14, xe2x80x94CR15R16R17 and xe2x80x94CaH(2a+1) (here, a is an integer from 1 to 5), R5 and R7 are independently xe2x80x94C(O)CR18xe2x95x90C(R19R20) or xe2x80x94CR21(R22R23), R6 is hydrogen or xe2x80x94CaH(2a+1) (here, a is an integer from 1 to 5), m is an integer from 1 to 5, n is an integer from 1 to 20, x is an integer from 1 to 15, and R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, R18, R19, R20, R21, R22, R23, R24 and R25 are independently hydrogen or xe2x80x94CbH(2b+1) in which b is an integer from 1 to 5, Rxe2x80x2 is hydrogen or CH3, and Rxe2x80x3 is hydrogen, xe2x80x94C(xe2x95x90O)CHxe2x95x90CH2 or xe2x80x94C(xe2x95x90O)C(CH3)xe2x95x90CH2.
In the lithium battery, the composition preferably further includes 0.1 to 5 parts by weight of at least one polymerization initiator selected from the group consisting of benzophenone, benzoyl peroxide, acetyl peroxide, lauroyl peroxide and azobisisobutyronitrile, based on 100 parts by weight of the composition.
In the lithium battery, the curing is preferably performed by at least one method selected from the group consisting of thermal polymerization, electron beam polymerization and UV polymerization, and the thermal polymerization is preferably performed at a temperature in the range from 60 to 100xc2x0 C.
Also, the lithium salt is preferably at least one selected from the group consisting of lithium perchlorate (LiCIO4), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium trifluoromethanesulfonate (LiCF3SO3) and lithium bistrifluoromethanesulfonyl amide (LiN(CF3SO2)2)
Further, the organic solvent is preferably at least one solvent selected from the group consisting of propylene carbonate, ethylene carbonate, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, vinylene carbonate, triglyme, tetraglyme and y-butyrolactone.
The electrode assembly may be a winding type and the case may be a pouch. Also, the separator may be formed of a polyethylene sheet, a polypropylene sheet or a combination thereof.